The present invention generally relates to engraving apparatus having a shaft with a stylus thereon for engraving cylinders commonly used in a gravure printing process and, more particularly, to an improved engraving apparatus including a control system for controlling the movement of the shaft and the engraving stylus and electronically damping the same.
Apparatus for engraving cylinders used in the gravure printing process are well known in the prior art. See, for example U.S. Pat. Nos. 4,451,856; 4,450,486; 4,438,460; and 4,357,633. The basic principle of engraving a gravure cylinder involves rotating a copper plated cylinder while actuating an electrically driven shaft having a tool or stylus thereon which cuts or engraves cells or lines into the copper surface. The engraving tool used to engrave the cells is normally a pointed diamond stylus. Other tools made of sapphire, carbide, cobalt steel, may also be used, but generally give shorter life, and due to wear, do not hold as consistent a point as diamond.
In the engraving of a gravure cylinder, an image pattern or information to be engraved is usually mounted on a copy cylinder, and the information is optically scanned when the engraving is being performed and sent to the engraving apparatus as a video signal. However, the information may be scanned and stored in computer memory, processed, and later used to engrave a cylinder.
During engraving, the video signal is superimposed with a drive signal of a substantially constant frequency to create an engraver drive signal for actuating the electrically driven shaft associated with the cutting stylus. The drive signal causes an angular oscillatory movement of the stylus to form or cut a succession of depressions which are referred to as cups or cells in the surface of the cylinder. The video signal, which is an indication of the measured density of each particular portion of the pattern being reproduced, will determine the cutting depth of the engraving stylus while forming each cell. Thus, during scanning of the pattern, a black part has a large density and will produce a video signal that will cause the engraving stylus to produce a deep cell. However, a white portion of the pattern will have little density and result in a video signal which will control the engraving stylus to produce a shallow cell.
While engraving, the stylus must make many cells in a cylinder, and therefore, must be operated at a very high speed. For example, in a typical 140 line screen, about 20,000 cells per square inch are required. More than 100 million cells are frequently required for a single large diameter gravure printing cylinder. Even with a forming rate of about 3,000 to 5,000 cells per second, several hours of time may be required to engrave a single cylinder.
During the engraving of a cylinder, variables associated with the operation of the engraving stylus can affect the amplitude of the angular oscillatory movement of the stylus resulting in variations in its performance. These variables may include: continuous changes in the loading, and hence, the cutting depth of the stylus; differences in hardness along the surface of the material being engraved; temperature changes in the magnetic drive mechanism resulting in a change in the magnetic field created therein; and, the affect of damping material employed with the engraving stylus. Variations in the amplitude of the angular oscillatory movement of the engraving stylus can cause "drift" of the engraving stylus resulting in the cells slowly becoming larger or smaller. As a consequence, the quality of the engraving performed by the stylus deteriorates.
Engraving apparatus in the prior art have not been capable of sensing and correcting such variations in the amplitude of the angular oscillatory movement of the engraving stylus. For this reason, a need exists for an engraving apparatus which employs a control system for sensing and correcting variations in the amplitude of the angular oscillatory movement of an engraving stylus.
Dampeners have been employed in the prior art for reducing undesirable transverse and rotational vibrations in the driven shaft and the stylus of an engraving apparatus. See for example, U.S. Pat. No. 4,438,460. It has been found, however, that at high speeds, the viscosity of the material comprising at least a portion of the dampener tends to change as the absorbed vibrational energy is converted to heat. As a result, the damping performed by the dampener tends to change as the viscosity of the material changes due to its rise in temperature. Accordingly, a further need exists for an engraving apparatus which employs a damping means which will have a substantially constant damping effect regardless of the speed at which the engraving stylus is operated.